The subject matter of the present invention relates generally to veneer lathe apparatus for peeling sheets of veneer from a rotating log by means of a lathe knife and associated nose bar and, in particular, to an electrical control system and method for automatic adjustment of lathe components which engage the log. The adjusted lathe components include the lathe knife pitch angle, the knife to nose bar spacing (referred to as the "knife gap"), and the radial position of the backup idler rolls or the core drive rolls. These lathe components are automatically adjusted by a computer control system in response to reduction of the diameter of the log as well as being provided with offset temperature adjustments in response to changes in the surface temperature of the log during peeling.
As shown in U.S. Pat. No. 4,708,180 of R. A. Browning, et al., issued Nov. 24, 1987, it has heretofore been the practice to provide a veneer lathe with automatic adjustment of the lathe knife pitch angle by a mechanical control including a cam follower roller on the knife carriage which rolls along a pitch rail cam member. The slope of the pitch rail is set manually or by other mechanical adjustment means prior to the start of peeling, and the knife pitch is then adjusted entirely by movement of the cam follower roller along the surface of the pitch rail during peeling. In most cases, the pitch rail is linear with a constant slope. However, such roll may be provided with a custom profiled end portion of changing slope for small diameter logs as discussed in the Browning patent. Unfortunately, such a custom-made pitch rail having an end portion of different slope does not apply to all species or types of wood. In addition, manual adjustment of the pitch rail to set the slope angle at the start of the peel is time consuming and inaccurate. The above-cited Browning patent also discloses the use of an automatic control means employing a computer for adjusting the knife gap between the lathe knife and a nose bar roll engaging the log immediately in front of the knife in response to changes in the log diameter and knife pitch during peeling. However, there is nothing in this patent which teaches that the pitch angle of the knife or the knife gap or other lathe components which engage the log, such as the backup idler rolls or core drive rolls, should be adjusted automatically in response to changes in log temperature by an electrical control system including a digital computer in accordance with tables of the component adjustments stored in the memory of such computer in the manner of the present invention.
During the veneer peeling process, better quality veneer is usually produced when the lathe knife pitch angle and knife gap are varied as the radius of the log diminishes during peeling. Also, the quality of the veneer is improved if the log is heated above room temperature by soaking in hot water or steam for at least several hours before peeling. Typically, the knife pitch angle, which is the angle the front cutting edge of the knife makes with a vertical plane passing through the tip of such knife, varies from a positive angle or "lead" where the knife is tipped away from the log for logs of large diameter to a negative pitch angle or "heel" where the knife blade is inclined toward the log for small diameter log blocks. The difficulty is that the manner in which the knife pitch angle and knife gap should be varied during peeling is different for different species or types of wood and for different temperatures of the log block during peeling. This requires frequent manual adjustment of the slope of the pitch rail and replacement of any custom profile pitch rail with another profile when such rail is provided with an end portion of changing slope. These problems are overcome in the present invention by electrical control of the lathe knife pitch angle using a digital computer having digital data representing different profiles of pitch angle adjustment corresponding to different wood species logs at different log temperatures, provided as a plurality of different knife pitch tables stored in the memory of the computer.
In the electrical control system of the present invention, the computer is employed to control adjustment of the knife pitch by adjustment of the vertical position or height of the cam follower roller on the knife carriage to support such carriage at different heights above the pitch rail. In the preferred embodiment, the height adjustment of the cam follower roller is accomplished by means of an eccentric which is rotated by a lever arm connected to the piston rod of a pitch adjustment cylinder operated by a servo valve in response to a pitch control output signal of the computer which changes with the diameter and the surface temperature of the log during peeling. The cylinder contains a transducer which produces a piston rod position signal corresponding to such height which is fed back to the computer to provide a closed loop feedback control in order to accurately set the pitch angle within 0.01 degree as a function of the horizontal position of the main knife carriage along the pitch rail and of the log surface temperature during peeling.
The control system of the present invention can also be used to control adjustment of the knife gap by means of another eccentric. This eccentric is also rotated by a lever arm connected to the piston rod of a gap adjustment cylinder operated by a servo valve in response to a gap control output signal of the computer which changes with the diameter and surface temperature of the log during peeling.
The present invention has the advantage that different pitch angle profiles for adjustment of the lathe knife pitch angle and different knife gap adjustment curves are easily defined and revised by changing the data entries stored in the computer memory. In addition, the computer enables quick selection of the appropriate pitch profile from a plurality of pitch profile tables stored in such memory. The same is true of different knife gap adjustment curves provided by different knife gap tables stored in the computer memory. Also, the ability to develop more optimum pitch profiles and knife gap curves for different wood species and different log temperatures is made possible. As a result, the quality of veneer is improved, and the production output of the veneer lathe is increased.